The document discusses leucocytes (white blood cells). It describes the classification of WBCs into granulocytes and agranulocytes. Granulocytes include neutrophils, eosinophils, and basophils which are involved in phagocytosis and immune responses. Agranulocytes include lymphocytes and monocytes. Lymphocytes provide humoral and cellular immunity while monocytes circulate in blood and differentiate into tissue macrophages. The reticuloendothelial system comprised of macrophages is also described which acts as a generalized phagocytic system in the body.